There is no sanctioning body for 3 gun or Multi gun. 3 Gun Nation the TV show has probably the most comprehensive unification of the sport but they aren't creating rule books just yet. They are trying to grow their show by growing the sport. The USPSA has the most comprehensive rule book if you want to learn the rules.
I agree with everyone above that said shooting some USPSA, IDPA, icore, GSSF, SCSA or any if the action pistol sports will better prepare you for 3 gun. All of those will make you more proficient with 1/3 of the guns you will use in 3 gun.they will all also teach u the safety rules which are first and foremost on the list. With that said I don't think you should wait to start 3 gun until u have experience elsewhere. I shoot with and work as a range officer for first time shooters at our 3 gun matches. I see plenty of Guys and gals and kids do great when their first shooting competition is 3 gun.
Plus there are so many things you will learn every match for pretty much forever that the longer you wait to start the further behind you will be and the more gun you will miss out on.